In reviewing an article about the proposed legislation "that would require the State Highway Administration to create a permit program allowing individuals and organizations to display American flags in traffic roundabouts", I noticed the following quote:
"[The SHA] also expressed concern that the bill is not 'content neutral' — it would allow for flags to be flown but not other items — and would raise 'a likely challenge under the First Amendment of the U.S. Constitution.'"
In fact, if I were you, I would keep government out of it. SHA can remove your flags at its own peril. No one is going to pass a law prohibiting the removal of the Northrop Team.